It is estimated that between 3.5 and 10% of the world population is afraid of needles, also called trypanophobia, and it is much more common in people between the ages of 5 and 17.

In this article we are going to examine the most common hypodermic needles used to vaccinate fish, their origin and recommendations for use.

A hypodermic needle (from the Greek ὑπο- (hypo- = under), and δέρμα (derma = skin)), is basically a very thin, hollow tube with a sharp, bevelled tip. [Wikipedia]

It is typically used in conjunction with a syringe to inject substances into a body (e.g. saline, vaccines, or medicinal fluids) or to extract fluids from a body (e.g. blood). In the case of fish, they are mostly used to draw blood or inject vaccine (intraperitoneally, and some intramuscularly).

By Bronami - Own work, CC BY-SA 4.0

The first hypodermic needle was designed by the Irish doctor, Francis Rynd, and on 3 June 1844, he made the world's first subcutaneous or hypodermic injection. Using a trocar (a punch covered with a cannula), he made a puncture in his patient's gum, allowing the passage of morphine through the cannula to under the skin through the effect of gravity (Rynd, 1845).

Image 1A & 1B. Dr Rynd's device (Rynd, 1861) (52 of 568).

Scottish doctor, Alexander Wood, added a plunger syringe to the hypodermic needle, thus inventing the modern hypodermic syringe in 1853. This syringe allowed doctors to inject solutions more quickly, and to perform intravenous injections. It also allowed blood samples to be taken to diagnose a growing range of disorders and diseases.

A glass syringe said to have been used by Dr Alexander Wood.

Today, the same type of hypodermic needles are still used, but now they are much more robust, made of higher quality materials (stainless steel) and have greater resistance. Today's needles have sharper bevels that offer longer life and less pain when penetrating the skin.

In a recent study, the degradation of needles that are reused in intramuscular vaccination in piglets has been evaluated (Owen et al., 2022). To do so, they used 21 G 5/8” needles, and after the application of 1, 12, 36 and 100 punctures, they evaluated the force necessary to penetrate the skin (N). They found significant differences in the force required to penetrate the skin as the number of punctures with the same needle increased, suggesting rapid bevel wear. This was also confirmed by electron microscopy (SEM), as can be seen in these images from the study:

SEM images of 2 unused 21G needles (Needle 1 and Needle 2). Owen et al., 2022.

SEM images of 2 needles used 1 time, and the force necessary for them to penetrate the skin (A-D); and another two needles used 36 times, and the force necessary for them to penetrate the skin (E-H). Owen et al., 2022.

SEM images of 2 needles used 1 time, and the force necessary for them to penetrate the skin (A-D); two needles used 12 times, and the force necessary for them to penetrate the skin (E-G); two needles used 36 times, and the force necessary for them to penetrate the skin (H-K); and two needles used 100 times, and the force necessary for them to penetrate the skin (L-O). Owen et al., 2022.

The bevel degradation seen in the SEM images is evident. For intraperitoneal vaccination of fish, thinner needles, size 23G or 22G, are commonly used. With thinner needles, less force is required to penetrate the skin, but they are also more delicate or less robust.

A degraded bevel can lead to bleeding and muscle tears. Furthermore, from a biosecurity point of view, the more the same needle is used, the greater the risk of contamination between vaccinated fish.

Important: do not reuse a needle already used a few hours before. The risk of contamination of the injection site is very high.

To choose the right needle for IP vaccination, it is important to pay attention to three aspects:

• The type of connection: the pistons most commonly used for IP vaccination have a Luer Lock connection, so the needle has to have the same type of connection.
   
• The calibre or G (Gauge): this refers to the diameter of the needle, and the higher the G, the smaller the diameter. It is important to use the smallest diameter possible so that the damage created by penetrating the skin is minimal, while still allowing smooth passage of the vaccine through the needle. These are the G most used in IP vaccination:

• The length: indicates the depth at which the vaccine will be injected. This parameter is very important; the vaccine could be injected into the internal organs, or into the abdominal wall, if it is too short; in both cases, the efficacy of the vaccine would be reduced, or we could have secondary effects in the animal, which could cause the death of the animal.

The appropriate length is that which, when vaccinating, exceeds the abdominal wall by 1-2 mm, within the peritoneum.

References

Rynd F. Description of an Instrument for the Subcutaneous Introduction of Fluids in Affections of the Nerves. Dublin Quarterly Journal of Medical Science. 1861; 32: 13

Rynd F. Neuralgia – introduction of fluid to the nerve. Dublin Med Press 1845; 13: 167–168

Owen, K.; Blackie, N.; Gibson, T.J. The Effect of Needle Reuse on Piglet Skin Puncture Force. Vet. Sci. 2022, 9, 90.